NISAL (Numerically Integrated Safety Assurance Logic)-based systems for use in railroad signal controlling devices have been known, and are described, for example, in U.S. Pat. No. 4,956,779 assigned to the assignee of the present application, being incorporated herein by reference. NISAL is a numerically based safety principle which relies on mathematical algorithms to verify the accuracy of a signal, thereby rendering the signal "vital". A method and apparatus for effecting vital functions without the use of vital hardware, is described in U.S. Pat. No. 4,831,521, assigned to the assignees of the present application, being incorporated herein by reference. The latter reference describes processors and systems which are known as vital processing interlocking systems, which are rendered vital by means of numerically based safety principles. However, when it becomes necessary to link together such systems a vital link between such processors has to be established and maintained to avoid the misdirecting of signals, and the transmittal of signals which include corrupted data. Therefore, there has been a need for transmitting signals between vital processors using a vital communications link that does not rely on vital hardware, communication protocols or error free software.
Accordingly, it is a fundamental object of the present invention to provide a point-to-point serial communications network for exchanging vital state parameters between interlocking vital processor control systems using numerically based safety principles in place of conventional vital link hardware, such as that disclosed in U.S. Pat. No. 4,956,779.
Another principal object is to enforce link vitality between processors by having the link processors exhibit a specific numerical relationship with one another.
Another object is to include a dynamic offset embedded within the link messages.
A further object is to provide a software arrangement which generates the link application data needed by the hardware to execute link assignments.